Add negative guard

 Bug: 15631600

Change-Id: Idd7740f58876e73694fafb8ef55ebaff511f6dca
diff --git a/libs/hwui/Interpolator.cpp b/libs/hwui/Interpolator.cpp
index 1f84b86..fc0e8a0 100644
--- a/libs/hwui/Interpolator.cpp
+++ b/libs/hwui/Interpolator.cpp
@@ -112,6 +112,10 @@
     int i1 = (int) ipart;
     int i2 = MathUtils::min(i1 + 1, mSize - 1);
 
+    LOG_ALWAYS_FATAL_IF(i1 < 0 || i2 < 0, "negatives in interpolation!"
+            " i1=%d, i2=%d, input=%f, lutpos=%f, size=%zu, values=%p, ipart=%f, weight=%f",
+            i1, i2, input, lutpos, mSize, mValues, ipart, weight);
+
     float v1 = mValues[i1];
     float v2 = mValues[i2];